A Regional Analysis of Clumped Isotope Geochemistry to Define the Timing of Creation of Micro-Porosity in a Lower Cretaceous Giant Reservoir

Abstract

Abstract Clumped isotope geochemistry of calcite minerals measures temperature independently of the isotopic composition of the precipitating water, allowing in combination with basin modeling to define the depth and timing of major diagenetic events. This technique has been applied to a regional selection of samples from the most prolific Thamama reservoir unit in Abu Dhabi in an attempt to further constrain the creation of the micro-porous system that volumetrically dominates the pore system of the reservoir. A total of 38 samples from 8 crest wells in 8 different fields from onshore Abu Dhabi were analyzed, indicating precipitating temperatures of 47°C to 110°C, and diagenetic water isotopic composition of 0.2‰ to 5.6‰ δ18O. Carbon and Oxygen stable isotope values are in line with existing values for this reservoir (−9.17 to −5.27 δ18O; 2.99 to 4.10 for δ13C). From the same cores and at the same depth as the 38 samples, thin sections were created for a semi-quantitative petrographic analysis to provide further insight on Δ47 temperature variations. Samples with a higher volume of intraclasts have lower temperature of precipitation while samples with higher volume of diagenetic cement show higher temperatures of precipitation. Cutoffs were applied to filter out these early and late diagenetic overprints. Using the existing basin model, Δ47 temperatures translate into a depth for diagenesis of between 610 and 1370 m and an Upper Cretaceous age. Both the range of temperatures and depths reflect differential transformation of the individual structures during the late Cretaceous caused by the emplacement of the Semail Ophiolite. This is in agreement with a recently proposed regional burial diagenetic model (Vahrenkamp et al., 2014). The isotopic composition of the diagenetic fluids indicate the presence of heavy burial diagenetic brines on a regional scale. The results show that the effects of deep burial diagenesis are present throughout the Abu Dhabi region and that a strong regional burial flow system is controlling the evolution of the poro-perm system. An understanding of this large scale fluid-flow system is important in resolving issues related to the regional distribution of reservoir properties.